Problem: 3 notebooks cost $4.38. Which equation would help determine the cost of 7 notebooks?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 7 notebooks. Since 7 notebooks cost $x$ , we have the following proportion: $\dfrac{7}{x}$ We can write the fact that 3 notebooks cost $4.38 as a proportion: $\dfrac{3}{\$4.38}$ The cost changes along with the number of notebooks purchased, and so the two proportions are equivalent. $\dfrac{7}{x} = \dfrac{3}{\$4.38}$